1. Field of the Invention
The present invention relates to a cylindrical dynamic pressure bearing having grooves for generating dynamic pressure on an inner peripheral surface thereof and particularly to a dynamic pressure bearing which is formed by the injection molding process.
2. Description of the Prior Art
Conventionally, as a dynamic pressure bearing of the above-mentioned type, there has been a dynamic pressure bearing which is made of a plastic material and formed by the insert molding method using a shaft as a male insert.
In order to mold a plastic material with accuracy by the insert molding method, it is important for the plastic material to have a good flowability and small shrink characteristics. The good flowability of plastics is an important factor in forming shallow dynamic pressure generating grooves of a depth of several micrometers with dimensional accuracy in a cylindrical dynamic pressure bearing, while the small shrink properties are an important factor in achieving a good cylindricity of the inside diameter of the cylindrical dynamic pressure bearing.
Unfortunately, it has been difficult to achieve a dimensional accuracy required for the dynamic pressure bearing no matter how the molding conditions are set.
In a dynamic pressure bearing formed of a plastic material such as polyamide or polyacetal having a good flowability and yet great shrink properties, a significant warpage occurs in the inner peripheral surface of the bearing. As a result, the bearing will not have a good cylindricity. FIG. 1 shows the warpage W of the inner peripheral surface.
FIG. 2 and 3 respectively show inner peripheral configurations of injection-molded dynamic pressure bearings formed of a plastic material such as PC (polycarbonate) or LCP (liquid Crystal polymer). Because such plastics have small shrink properties and yet an inferior flowability, the following phenomenon occurs. That is, as the grooves are farther from a gate of the injection mold, the grooves become shallower, as shown in FIG. 2. Otherwise, as the groove are farther from the gate, the inside diameter becomes smaller, as shown in FIG. 3. The cylindricity thus deteriorates.